This invention relates to an alternating current (AC) driven laser diode array, to an AC power supply for a laser diode array, and to a method of energizing such a laser diode array with AC power.
Generally, a semiconductor laser array utilizes a stack or array of semiconductor devices, such as a pn junction diodes, which when energized (or pumped) to a level such that the lasing threshold of the diodes is exceeded (i.e., when the current density within the diode is such that a light pulse can travel a round trip within the resonating cavity of the diode), coherent radiation is emitted from the front, partially-reflecting facet of the laser diode. More particularly, a semiconductor laser array typically comprises a stack of pn junction diodes with each of the diodes being a small block of a suitable crystalline semiconductor material such as germanium, silicon, gallium arsenide (GaAs), gallium aluminum arsenide (GaAlAs), gallium arsenide phosphide (GaAsP), indium gallium arsenide phosphide (InGaAsP), or other suitable semiconductor material. Each of the laser diodes has a fully reflecting back facet and a partially reflecting front facet defining an optical resonating cavity (referred to as the Fabry-Perot cavity) with the front and rear facets being referred to as the Fabry-Perot surfaces of the laser diode.
In a semiconductor laser, the fundamental light producing mechanism in the diode is the recombination of electrons and holes when a conduction-band electron is captured by a valence-band hole. Pumping of the laser diode is accomplished by the injection of electrons across the pn junction. More specifically, the laser diode is positively biased with the p-side of the diode positive. At low current levels within the diode, the electrons recombine with the holes spontaneously emitting radiation in all directions. At higher current levels, an inverted population of carriers is achieved yielding a positive gain in the lasing region. When the current density exceeds the lasing threshold of the diode, a light pulse is able to traverse a round trip within the optical resonating cavity of the laser diode without attenuation and a coherent pulse of light will be emitted from the partially reflecting front facet of the diode.
The threshold current density of a laser diode is a strong function of temperature. Large current densities are required to achieve an inverted population of carriers. For example, at room temperature, current densities ranging between about 8,000 and 40,000 ampers/cm..sup.2 may be required. Above 100.degree. K., the threshold current density is approximately proportional to the cube of the temperature. To minimize the self-generated Joule heating effects, laser diodes are typically periodically pulsed with each of the energized pulses having an energization time ranging between about 100-400 nanoseconds at a frequency ranging between about 1K and 10K Hz.
Certain semiconductor laser materials, such as a homojunction GaAs diode, have a relatively low room temperature efficiency (e.g., less than about 1%), but at cryogenic temperatures (e.g., 77.degree. K.), these materials have efficiencies as high as 60%. This laser diode structure is usually cryogenically cooled. Single-heterostructure laser diodes, such as GaAlAs, have appreciably lower threshold current densities and have thus made room temperature (i.e., non-cryogenically cooled) diode lasers practical.
Double heterostructure laser diodes utilize a second GaAlAs layer at the pn junction so as to provide a wave guide within the diode, so that even lower threshold currents result, thus making continuous wave (CW) room temperature diode lasers possible.
Continuous wave or pulse operated laser diodes have many applications in fiber optic data transmission sensors, voice communications, and in ranging devices. In certain signalling applications where peak power output is more important than average power output, room temperature laser diode arrays are often times employed. Such laser diode arrays are shown in U.S. Pat. Nos. 3,514,715, 3,878,556, 3,962,655, 4,032,802, 4,069,463, and 4,298,869.
As previously mentioned, room temperature laser diodes are generally less efficient than cryogenically cooled laser diodes, thus requiring substantial amounts of heat to be dissipated. This is usually accomplished by incorporating heat sinks between the diodes of the array with the heat sinks conducting away heat from the diodes. Reference may be made to such U.S. Pat. Nos. 3,436,603, 3,694,703, and 3,921,201, which illustrate various semiconductor laser diode array cooling arrangements. Often times, the heat sinks for these laser diode arrays have fins or other heat dissipation surfaces over which air or other heat transfer mediums may be circulated to carry away the dissipated heat.
Typically, each of the front facets of the laser diodes in a stack or array is in communication with a respective fiber optic fiber which in turn are bundled and fed to a suitable optical integrator or the like, depending on the particular application for the diode laser. Also, the stacks or arrays of laser diodes may be arranged in a circular configuration centered generally on the fiber optic bundle, with the emitting facets of the various laser diodes of the diode arrays being directed generally radially inwardly so that a large number of the laser diode arrays can be compactly arranged with respect to the optical integrator, and such that the length of the fiber optic fibers from the diodes to the optical integrator is minimized.
A variety of pulsed power supplies have been developed for driving or pumping laser diodes and laser diode arrays. These power supplies include delay line SCRs, capacitor SCRs, SCR pulse transformers, avalanche transistors, transistors, mechanical relays, and gas tube circuits. However, these systems supply DC power to the laser diode or diode arrays. Generally, each of these prior art power supplies includes a trigger circuit, electronic switches, protective circuits, and a primary power source. The limitations on voltage and/or current of the electronic switching elements (e.g., SCRs or transistor switches) require that the laser diodes in a multiple diode array be electrically connected in series-parallel circuits. Thus, as the number of diodes increases, impedence mismatching problems may arise.
A conductively cooled, DC-powered laser diode array was developed which operated with a continuous wave (CW) output. Also, so-called burst modulated diode pumps have been developed in which a laser diode array is powered by a square or rectangular wave form. However, the construction of the conductively cooled diode arrays had such a high value of electrical capacitance that it was not practical to use the above-mentioned burst modulation pumping technique.